This invention relates to a loop mirror filter that can be used in optical amplifiers of the kind which employ spontaneous emission, such as, for example, erbium-doped fiber amplifiers.
Optical amplifiers are used in optical telecommunications systems at the transmitter, receiver, and as repeaters or in-line amplifiers. Such optical amplifiers typically use an optical fiber doped with a rare earth element, for example erbium, to provide stimulated emission which is used to amplify the signal. Unfortunately, spontaneous emission occurs, is amplified along with the input signal, and appears as noise in the output signal, reducing the signal-to-noise ratio. It is desirable to improve signal-to-noise ratio by removing or substantially reducing such amplified spontaneous emission (ASE) noise.
In an article entitled xe2x80x9cIn-band Amplified Spontaneous Emission Noise Filtering with a Dispersion-imbalanced Nonlinear Loop Mirrorxe2x80x9d, OFC 1999, William S. Wong et at. disclosed an experimental set up which reduced noise by means of a non-linear optical loop mirror, i.e. formed by two fibers of different types, and with different lengths. The set up comprised an EDFA and an external ASE generator which injected the input signal and noise, respectively, via a 3-dB coupler and a circulator, into a dispersion-imbalanced loop mirror formed by another 50/50 coupler, two lengths of xe2x80x9cLucent TrueWavexe2x80x9d optical fiber having different anomalous dispersion characteristics, and a polarization controller. The polarization controller was used to null loop transmissions at low power so as to minimize the effects of natural birefringence. The signal from the loop mirror was ported to a lightwave receiver which performed bit error rate measurements. A disadvantage of Wong et al.""s design is that the loop mirror is formed by a transmission line, which is about 24 km long. While this might be acceptable in an experimental set-up, it is not practical or useful in a real system. Also, Wong et al. must use high power levels to exploit the non-linearity of the fiber used in the loop in order to achieve transmission of the signal.
Japanese patent application No. 11087822A, published Mar. 30, 1999, owned by Samsung Electron Co. Ltd., and naming Wang Hwang Seong-Taek as inventor, disclosed an arrangement for providing a high small-signal gain in an erbium-doped fiber amplifier. Seong-taek disclosed an EDFA comprising a circulator, a wavelength selective coupler, a laser diode pump and an erbium-doped fiber connected to a loop mirror by a coupler. The amplified signal from the EDF is reflected by the loop mirror so that it passes through the EDF again, but in the opposite direction. A disadvantage of this arrangement is that, although gain might be increased, overall signal-to-noise ratio will deteriorate because the signal will pass through the amplifier twice, incurring spontaneous emission noise penalties each time.
The present invention seeks to eliminate or at least mitigate these disadvantages of known systems.
According to one aspect of the present invention, an optical amplifier comprises an amplifier section for amplifying an input signal by means producing spontaneous emission amplified signal and a loop mirror filter for filtering the amplified signal to remove amplified spontaneous emission.
Preferably, the loop mirror filter comprises a 3-dB coupler and a loop of optical fiber having an active section. Optionally, the loop mirror filter includes a circulator for directing the amplified signal and the filtered signal to and from the loop of fiber. Preferably, the loop of optical fiber is relatively short, say less than 5 meters.